[CYCLE_MONKEY]

The use of copper will merely pit the aluminum where the metals are in contact. The damage will be minimal, but why invite it?

Excellent point about the anodic/cathodic properties, especially in an area likely to see a lot of moisture (enabling/enhancing the corrosion as it's the medium). For our aluminum control section housing that I redesigned (JSOW missile), at first we had to coat with epoxy primer every stainless threaded insert (helicoil type) we used to prevent corrosion. We had a dedicated Engineer who did nothing but look at materials and corrosion issues. However, after new research came out in the mid '90's and was given to us by TI, it was found that, basically, the electrical potential of the dissimilar metals mattered. In other words, if the inserts were a small percentage of mass compared to the aluminum housing, the corrosion was not an issue. If it was a stainless casting with aluminum inserts (for the sake of argument) then the inserts would get eaten up in short order.

In this case, since the copper gasket is a very small mass compared to the engine castings, I'd say it's probably not an issue, especially since there's almost certainly a thin layer of oil between the 2 as a barrier. But since it has the "potential" (pun intended:laugh to cause light corrosion on the mating interface, why even chance it? The issue I'd be more worried about is that the torque value was designed based on how much pressure it would take to crush the softer aluminum, and with a copper washer of unknown value, you might end up getting either not enough or too much clamping force for a certain torque setting...possibly leading to stripping threads or the bolt falling out.

In short, me, I'd use stock washers and make sure they're oriented correctly and be worry-free...